Michael Ashcroft once said that from the moment he first met William Hague, shortly after the Yorkshireman was elected Conservative leader in June 1997, they "enjoyed a certain mutual chemistry".

People who know both men say they are extraordinarily close: they spend a lot of time together, and their wives enjoy each other's company. Ashcroft admires Hague for what he describes as his brilliant analytical mind, his ability to concentrate while under pressure and his unflappability.

Hague, meanwhile, is grateful to Ashcroft, who remained loyal to the Conservatives after the catastrophe of the 1997 general election, holding out a financial lifeline to the party when so many others walked away.

That gratitude may explain the almost desperate language Hague employed when writing to Tony Blair in May 1999, begging the prime minister not to reject his friend's peerage: "He [Ashcroft] is … committed to becoming resident by the next financial year … this decision will cost him (and benefit the Treasury) tens of millions a year in tax … if you still feel unable to recommend his name for a working life peerage, or to arrange for these points to be put urgently to the (Honours Scrutiny) Committee, I would ask for an urgent meeting with you to discuss the matter further …"

Hague failed to secure a peerage for his friend on that occasion, and almost failed 12 months later, until he persuaded the tax-exile tycoon to give, in writing, his "clear and unequivocal assurance" that he would take up permanent residence in the UK.

Today, that promise has returned to haunt Ashcroft after a series of Freedom of Information Act requests forced him to disclose the truth about his non-dom status.

But that disclosure has left Hague exposed. For the next 48 hours – until today – Hague was forced to dodge a series of questions about what he knew, and when.

But what Hague was unable to evade was the charge that his willingness to believe Ashcroft's assurances raised serious questions about his judgment.

In the words of a Times leader on Tuesday, Ashcroft treated inquiries about his tax status "as a game to be played with a smirk", and by saying he accepted the assurances the tycoon had given, Hague, along with other senior Tories, had been left looking "evasive and weak".

It is unclear whether the mutual discomfort the two men are experiencing will do anything to dilute the "mutual chemistry". Indeed, evidence of just how strong a bond that chemistry has created over the years may be found in the small print of the parliamentary register of members' interests.

Ashcroft's support for the Conservatives and his appointment as party treasurer in 1998 is well known. The register shows that by March 1999, when Hague was still trying to establish some authority over a party in disarray, Ashcroft helped to provide air transport so the leader could fly across Britain.

The register also details a series of other flights and donations spanning several years. The following January Hague flew in Ashcroft's private jet around Florida, where the businessman has a home and is believed to spend much of his time, before flying on to New York. Friends of Hague say he is unabashed about his love of the high life. Ashcroft's Falcon 900 jet features "supple leathers, glistening veneers and deep pile carpet as standard", according to its French maker, Dassault. It boasts a galley that "houses all the essentials for a fine dining experience 45,000ft in the air".

When Hague stood down as leader after losing the 2001 general election, Ashcroft began making donations to his parliamentary office. In line with the rules at the time, the total sums have never been disclosed.

Soon after, Hague was being flown across the world in Ashcroft's jet, wholly owned by his company Flying Lion. He went to Prague in March 2006 for a meeting with the Civic Democratic party, a rightwing Eurosceptic party which has left the mainstream Euro group, followed by trips to Sudan, Israel, Jordan, China, Kazakhstan, Pakistan, Bahrain and Syria, all courtesy of Ashcroft.

Their relationship appears to have grown closer – and Ashcroft's support more generous – after David Cameron appointed Hague shadow foreign secretary in December 2005. The businessman began accompanying the politician on official shadow foreign secretary visits, giving him unique access to world leaders and prompting speculation that Ashcroft was being considered for a ministerial role in the Foreign Office in a future Tory government.

In April 2007, Hague flew 1,160 miles from Luton to Iceland in Ashcroft's jet. From there he and Ashcroft flew to Belize, the impoverished tax haven that has been Ashcroft's base for many years. The trip continued as both men flew to Panama, where Hague met and accepted hospitality from the foreign minister, Samuel Lewis Navarro. Then it was on to Brazil for more meetings before flying to the Falklands for a one-day visit to mark the 25th anniversary of the war.

In May 2008, after flying with Hague to Cairo, Ashcroft attended a meeting with Hosni Mubarak, the Egyptian president. Then last month it emerged that Hague and Ashcroft had flown to Cuba together without giving the British embassy any warning. "The first we knew about their visit was when Hague telephoned and said he was in Havana, and asked if he could meet our ambassador," an embassy official said.

The two men had flown in for a privately arranged meeting with Bruno Rodríguez, Cuba's foreign minister, and spent the night on board one of Ashcroft's yachts in Havana's Hemingway marina. While there, the Tories have confirmed, they again met Navarro, who flew into the country for a joint meeting with Cuban officials.Ashcroft also gained access to meetings with US officials when Hague flew to Washington in October last year. Hague was scheduled to meet officials including Barack Obama's national security adviser, General Jim Jones, the head of the Senate's foreign relations committee, John Kerry, Senator John McCain and the World Bank president, Robert Zoellick. The Tories refuse to say which meetings Ashcroft attended.

Less than four weeks ago, Hague was still evading questions about Ashcroft's tax status. Asked by Andrew Marr on BBC television: "Can you therefore tell me whether or not he pays tax in this country?", Hague responded: "Let me give you something that's not at all evasive and obfuscatory." He then failed to say whether Ashcroft pays tax in the UK.

While Hague has disclosed when he became aware of Ashcroft's non-dom status, there are a number of other matters he continues to evade.

Among them are the question of how he can justify Ashcroft having sat in the Lords as a non-dom for nine years when it is Tory policy to compel all those sitting in either house to be domiciled in the UK for tax purposes.

Questions for Hague

1. In 1999 you wrote to Tony Blair assuring him that Lord Ashcroft would change his tax status by the following financial year with the effect that he would pay "tens of millions a year in tax". Is it your understanding that Lord Ashcroft has indeed paid tens of millions a year in UK tax since becoming a peer?

2. Based on your assurance to Mr Blair, Lord Ashcroft would have paid over £100m in UK tax since becoming a peer. Do you believe it is possible he can have paid such a large sum solely on his UK earnings?

3. Do you believe that he misled you when he made a "solemn and binding" undertaking to take up permanent residence, but subsequently obtained non-dom status?

4. When did you become aware that Lord Ashcroft was not domiciled in the UK for tax purposes?

5. If you were aware that Lord Ashcroft was not domiciled in Britain between 2000 and 2010, did you make David Cameron aware of this?